The invention relates to the manufacture of vinylidene difluoride and trifluoroethylene copolymers, particularly terpolymers which are relaxors with electrostrictive properties.
In particular, the invention relates to copolymers including vinylidene difluoride (VDF) and trifluoroethylene (TrFE) associated with at least one monomer more reactive than they, such as 1,1-chlorofluoro ethylene (CFE).
The desired copolymers must have a relaxor property, meaning that the transformation barrier between the polar and non-polar phases is diffuse and that, at room temperature, a reversible change between these phases can be induced by an electric field with very little hysteresis. They must also have good mechanical elasticity allowing substantial longitudinal strain in an electric field.
In addition, these polymers must have electrostrictive properties, meaning that their strain is a function of the square of the electric field applied.
These characteristics must, in particular, translate into a high dielectric constant of at least over 50 and longitudinal strain induced by an electric field of at least 4%, as well as mechanical properties such as elasticity characterized by a Young's modulus of at least 0.3 GPa and good mechanical and electrical homogeneity.
Materials are known that have both strain levels induced by an electric field, high elastic energy densities, and high dielectric constants at room temperature. These materials are used in electromechanical devices that convert the electric energy into mechanical energy or vice versa. These materials are ceramics or polymers.
Ceramics, while having low hysteresis and a fast response rate, have the drawback of having low strain levels—approximately 0.1%.
The polymers on the market have the drawback of having low strains induced by an electric field corresponding to dielectric constants of less than 12.
U.S. Pat. No. 6,423,412 describes vinylidene difluoride polymers, particularly copolymers composed of x mol % of VDF and (100-x) mol % of TrFE where x is between 50 and 70 and terpolymers of vinylidene fluoride, trifluoroethylene, and hexafluoropropylene.
These polymers were cured at about 130° C. to 140° C. for about 16 hours, then irradiated in an oxygen-free atmosphere with an energy in the range of approximately 500 keV to 3 MeV in order to give rise to ferroelectric polymers with a relaxor nature which, at room temperature, have an electrostrictive strain of at least 4% when an electric field of at least 100 MV/m is applied thereto.
These polymers have a relaxor ferroelectric behavior and hence a slim polarization hysteresis loop in an electric field, coupled with high electric field breakdown strength.
However, the manufacture of the polymers described requires irradiation, and their properties are not acceptable for the desired applications of the invention.
U.S. Pat. No. 6,787,238 discloses terpolymers including vinylidene fluoride (VDF) associated with a second monomer chosen from trifluoroethylene (TrFE) or tetrafluoroethylene and with a third monomer, particularly 1,1-chlorofluoroethylene (CFE).
These polymers, when stretched past their initial length then cured at a temperature below their melting point, have induced strain, at room temperature, of at least 3% when exposed to an electric field of 100 megavolts/meter, a relative dielectric constant, at room temperature, of at least 40 at 1 kHz, and an elastic energy density at room temperature of at least 0.3 joules/cm3.
In this document, the variation in longitudinal strain induced by an electric field was measured as a function of field amplitude, at room temperature, for a terpolymer composed of 60 mol % VDF, 36 mol % TrFE, and 4 mol % CFE. For this polymer, a relatively high longitudinal strain (−4.5%) is induced in a field of approximately 150 MV/in. The polymers a high elastic modulus of approximately 1 GPa corresponding to an elastic energy density of about 1 J/cm3, far higher than that of piezoelectric materials. This terpolymer is composed of x mol % VDF, y mol % TrFE, and (100-x-y) mol % CFE where x is between 55 and 80 and y is between 15 and 40.
However, the terpolymers described in this document do not have the degree of performance required in the present invention.
A method for manufacturing dielectric terpolymers composed of 60 to 79 mol % vinylidene fluoride, 18 to 22 mol % trifluoroethylene, and 3 to 22 mol % chlorotrifluoroethylene is described in U.S. Pat. No. 5,087,679.
Other monomers such as tetrafluoroethylene and vinylidene fluoride can be added to the three basic monomers.
According to this method, polymerization is carried out in an aqueous suspension, i.e. with involvement of oil-soluble free-radical polymerization initiators and in the presence of a suspending agent.
The polymerization is carried out in an autoclave, preferably at a temperature of between 5° C. and 95° C., at a pressure of less than 50 kg/cm2.
The polymers obtained have a dielectric constant higher than 30, at room temperature. The highest dielectric constant shown in the examples is 46.7, representing a value lower than that sought in the invention.
In IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 11, No. 2, 2004, pp. 293-298, XP-002376851, F. Bauer, E. Fousson, Q. M. Zhang, and L. M. Lee disclose a method for manufacturing dielectric terpolymers according to which a mixture of VDF/TrFE/CFE monomers is loaded into an autoclave at constant temperature and pressure and a mixture of these constituent monomers is continuously reinjected during the polymerization reaction. However, the compounds obtained have neither good electric strength since their breakdown field value reaches only 250 V/μm nor good electrical and mechanical homogeneity.